Cambridge International AS and A Level Biology Practical 2: Biochemical Tests, Dilutions and Colorimetry

Biochemical Tests, Dilutions and Colorimetry develops Paper 3 and Paper 5 skills through controlled tests, quantitative concentration series, calibration curves and defensible identification of unknowns. The molecular basis of carbohydrates, proteins and lipids belongs in the theory hub. This practical note owns preparation, measurement, safety, records, inference and evaluation.

1. Build a valid comparison

Identify the independent variable, commonly concentration or sample identity, and a measurable dependent variable such as absorbance, transmission, time to a colour endpoint or final colour category. Standardise total volume, reagent concentration and volume, temperature, reaction time, pH, mixing and sample preparation.

Use enough independent-variable values to establish a pattern. Cambridge expects a minimum of five in practical decision-making. Replicate each value so variation can be assessed and a mean calculated. Random variation between repeats is evidence to report, not a reason to select the most convenient result.

A control reveals what happens without the tested component. A negative control might replace sample with distilled water. A positive control contains a known target and confirms that reagents and conditions can produce the expected response.

2. Prepare proportional dilutions

A proportional dilution mixes stock solution and diluent directly to reach each target concentration. The stock volume multiplied by stock concentration equals final volume multiplied by target concentration.

For a fixed final volume, calculate each stock volume, then add diluent until the final volume is reached. For example, to prepare 10 cubic centimetres of 40 percent solution from a 100 percent stock, use 4 cubic centimetres of stock and 6 cubic centimetres of diluent.

Prepare each concentration independently with clean equipment. Label tubes before transferring liquid. Use suitable syringes or pipettes and read scales at eye level. Small volumes can carry large percentage uncertainty, so choose stock and final volumes that the available apparatus can measure reliably.

3. Prepare serial dilutions

A serial dilution transfers a fixed proportion from one tube into a fixed volume of diluent in the next. If equal volumes are mixed at every step, each concentration is half the previous one. Other dilution factors follow from the volumes used.

Mix each tube thoroughly before transferring to the next, use a clean tip or rinsed transfer device and keep transfer volumes consistent. A mistake early in the sequence affects all later tubes, so serial dilution is efficient but propagates error.

Write concentrations for every tube before beginning. Remember that transferring liquid onward can change the volume remaining but not the concentration in the source tube.